The Vertical Structure of Tropical Convection and Its Impact on the Budgets of Water Vapor and Ozone

Abstract

Convective clouds in the Tropics that penetrate the boundary layer inversion preferentially detrain into a shallow outflow layer (2?5 km) or a deep outflow layer (10?17 km). The properties of these layers are diagnosed from a one-dimensional model of the Tropics constrained by observed mean temperature and water vapor profiles. The mass flux divergence of the shallow cumuli (2?5 km) is balanced by a mass flux convergence of evaporatively forced descent (downdrafts), while the mass flux divergence of deep cumulonimbus clouds (10?17 km) is balanced by a mass flux convergence of clear-sky radiative descent. The pseudoadiabatic temperature stratification of the midtroposphere (5?10 km) suppresses cloud outflow in this interval. The detrainment profile in the deep outflow layer is shifted downward by about 1.5 km from the profile one would anticipate based on undilute pseudoadiabatic ascent of air from the boundary layer. The main source of water vapor to most of the tropical troposphere is evaporative moistening. Below 12 km, evaporatively forced descent plays an important role in the vertical mass flux budget of the Tropics. This gives rise to a coupling between the water vapor and mass flux budgets, which, between 5 and 10 km, provides a constraint on the variation of relative humidity with height. Between 12 and 15 km, the observed relative humidity profile can be reproduced by assuming a simple first-order balance between detrainment moistening and subsidence drying. The mean ozone profile of the Tropics can be reproduced using a simple one-dimensional model constrained by the cloud mass flux divergence profile of the diagnostic model.